1. Field of the Invention
The present invention relates to a privacy communication device in radio communication for preventing any third person from tapping communications.
2. Description of the prior Ar
FIG. 1 is a block diagram showing a conventional privacy communication device as mentioned above. As shown in FIG. 1, a voice input signal "a" applied to a voice input terminal 6 is amplified in an input amplifier 7 and modulated in a ring modulator 8 producing double side band. An output signal "b" from the ring modulator 8 is inputted to a low-pass filter 9 for filtering the double side band to pass lower side band alone, and an output signal received from the low-pass filter 9 is amplified in an output amplifier 10 and applied as an output signal "c" to a privacy voice output terminal 11.
Then, a privacy voice input signal received from a privacy voice input terminal 12 is amplified in an input amplifier 13 and demodulated in a ring demodulator 14 producing double side band. An output signal "d" from the ring demodulator 14 is inputted to a low-pass filter 15 for filtering the double side band to pass lower side band alone, and an output signal from the low-pass filter 15 is amplified in an output amplifier 16 and then applied as a voice output signal "e" to a voice output terminal 17.
A carrier generator 25 applies a modulator carrier frequency signal 23 to the ring modulator 8 and a demodulator carrier frequency signal 24 to the ring demodulator 14.
FIG. 1 shows a system architecture of one of two privacy communication devices to communicate each other; the other not shown in the drawing also has the same architecture.
A privacy voice output signal from the privacy voice output terminal 11 shown in FIG. 1 is transmitted through a radio transmitter (not shown) to the privacy communication device remotely positioned not shown in the drawing and applied through a radio receiver to a privacy voice input terminal of the remote privacy communication device. Also, a privacy voice output signal from the privacy voice output terminal of the remote privacy communication device not shown in the drawing is transmitted from a radio transmitter built in the remote privacy communication device through a radio receiver (not shown) of the privacy communication device shown in FIG. 1 to the privacy voice input terminal 12.
Then, the operation of the devices will be described. First, the voice input signal "a" inputted from the voice input terminal 6 is amplified by the input amplifier 7 and applied to the ring modulator 8. From the amplified voice input signal "a", the ring modulator 8 produces two side bands "b", upper and lower bands with the center of the modulator carrier frequency signal 23 received from the carrier generator 25. The low-pass filter 9 pass only the lower band of the upper and lower bands, and the signal is amplified in the output amplifier 10 and outputted as the privacy voice output signal "c" from the privacy voice output terminal 11.
The privacy voice input signal inputted from the privacy voice input terminal 12 is amplified by the input amplifier 13 and applied to the ring demodulator 14. From the amplified voice input signal, the ring demodulator 14 produces two side bands "d", upper and lower bands with the center of the demodulator carrier frequency signal 24 received from the carrier generator 25. The low-pass filter 15 passes only the lower band of the two side bands, upper and lower bands. The signal is amplified in the output amplifier 16 and outputted as the privacy voice output signal "e" from the voice output terminal 17.
The modulator carrier frequency signal 23 and the demodulator carrier frequency signal 24 applied to the ring modulator 8 and the ring demodulator 14, respectively, are identical in frequency.
FIGS. 2 (a)-(e) are diagrams showing frequency spectrums of the signals "a" through "e" produced by the conventional privacy communication device. FIG. 2 (a) shows a frequency spectrum of the voice input signal applied to the voice input terminal 6, FIG. 2 (b) shows a frequency spectrum of the output of the ring modulator 8, FIG. 2 (c) shows a frequency spectrum of the privacy voice output signal outputted from the privacy voice output terminal 11, FIG. 2 (d) shows a frequency spectrum of the output of the ring demodulator 14, and FIG. 2 (e) is a frequency spectrum of the voice output signal from the voice output terminal 17.
As shown in FIGS. 2 (a)-(e), the ring modulator 8 produces two frequency spectrums of the upper and lower bands shown in FIG. 2 (b) of the voice input signal "a" shown in FIG. 2 (a).
Then, the low-pass filter 9 passes the lower band alone of the two frequency spectrums of the upper and lower bands produced by the ring modulator 8 and produces the privacy voice output signal "c" through the output amplifier 10. The frequency spectrum of the privacy voice output signal "c" has a profile which is the inversion of a profile of the frequency spectrum of the voice input signal "a".
Also, the ring demodulator 14 produces the two frequency spectrums in FIG. 2 (d) of the upper and lower bands of the privacy voice input signal "c" amplified by the input amplifier 13.
Then, the low-pass filter 15 passes the lower band alone of the upper and lower bands produced by the ring demodulator 14, and the voice output signal "e" from the low-pass filter 15 through the output amplifier 16 is outputted from the voice signal output terminal 17. The voice output signal "e" has a profile which is the inversion of a profile of the frequency spectrum of the privacy voice input signal "c" and which is identical with a profile of the frequency spectrum of the voice input signal "a".
As has been described, a frequency spectrum on the transmitting side is applied to the receiving side after it is inverted in the ring modulator 8 and low-pass filter 9 on the transmitting side, while the frequency spectrum received on the receiving side is inverted again in the ring demodulator 14 and low-pass filter 15 on the receiving side to reproduce the frequency spectrum of the transmitting side.
The ring modulator 8 and the ring demodulator 14 can be formed of the identical circuitry, and they carry out modulation and demodulation in accordance with carriers of the identical frequency. FIG. 3 is a circuit diagram showing an exemplary architecture of the ring modulator 8 and ring demodulator 14. As shown in the diagram, a voice input signal or a privacy voice input signal is inputted from a voice signal input terminal 26, while the modulator carrier frequency signal 23 or the demodulator carrier frequency signal 24 is inputted from a carrier signal input terminal 27. The voice signal input terminal 26 is connected through analog switches 28 and 30 to a negative phase input terminal "-" and a positive phase input terminal "+" of a differential amplifier 33, respectively.
A ground level is connected through analog switches 29 and 31 to a negative phase input terminal "-" and a positive phase input terminal "+" of the differential amplifier 33, respectively.
Moreover, under the control of the modulator carrier frequency signal 23 or demodulator carrier frequency signal 24 received from the carrier signal input terminal 27, the analog switches 28 and 31 become conductive or nonconductive, while under the control of the modulator carrier frequency signal 23 or demodulator carrier frequency signal 24 inverted by a logic inverter 32, the analog switches 29 and 30 become conductive or nonconductive.
Output of the differential amplifier 33 is applied to a modulated/demodulated signal output terminal 34.
The operation of the ring modulator 8 or ring demodulator 14 is as follows: When the modulator carrier frequency signal 23 or demodulator carrier frequency signal 24 inputted from the carrier signal input terminal 27 is at high level, a voice signal from the voice signal input terminal 26 is applied through the analog switch 28 to the negative phase input "-" of the differential amplifier 33 while a ground potential is applied through the analog switch 31 to the positive phase input "+" of the differential amplifier 33.
On the contrary, when the modulator carrier frequency signal 23 or demodulator carrier frequency signal 24 inputted from the carrier signal input terminal 27 is at low level, the voice signal from the voice signal input terminal 26 is applied through the analog switch 30 to the positive phase input "+" of the differential amplifier 33 while the ground potential is applied through the analog switch 29 to the negative phase input "-" of the differential amplifier 33.
For the reason, when the modulator carrier frequency signal 23 or demodulator carrier frequency signal 24 inputted from the carrier signal input terminal 27 is at high level, the amplitude of the voice input signal from the voice signal input terminal 26 deviates from the reference of the ground potential to negative side, contrarily, when the modulator carrier frequency signal 23 or demodulator carrier frequency signal 24 is at low level, the amplitude of the voice input signal deviates from the reference of the ground potential to positive side. In this way, the voice input signal is modulated in accordance with carrier, and the signal having the frequency of the carrier suppressed is taken out from the modulated/demodulated signal output terminal 34 as output of the ring modulator 8 or ring demodulator 14.
The conventional privacy communication device is configured as previously mentioned, and therefore, there arises the problem that communications through a plurality of privacy communication devices are easily tapped by any third person who gets the modulator carrier frequency signal 23 and demodulator carrier frequency signal 24 which is generated by the carrier generator 25 and which is employed in common in the plurality of privacy communication devices.